With the development of the Ethernet technology, operators can use Ethernet technologies in a Metropolitan Area Network (MAN) directly to provide various services. The development of diversified services spurs the Ethernet to evolve to carrier-class Ethernets of higher reliability. In this process, the Ethernet needs to interwork with other networks. In the conventional art, the ATM network is interconnected with the Ethernet by bearing Ethernet services on the ATM network to implement Local Area Network Emulation (LANE) over ATM. With the Ethernet evolving to a core of the MAN, the ATM network needs to be connected into an Ethernet to implement ATM emulation.
ATM is a cell-based communication technology, in which all data switching units are based on an information block of a fixed length, namely, a cell. Each cell is 53 bytes in length, and consists of 48-byte cell payload and 5-byte cell header.
Table 1 shows the structure of an ATM cell header of a Network/Network Interface (NNI).
TABLE 187654321VPIVPIVCIVCIVCIPTICLPHEC
As shown in Table 1, an ATM cell header contains: 12-bit Virtual Path Identifier (VPI), 16-bit Virtual Channel Identifier (VCI), 3-bit Payload Type Identifier, 1-bit Cell Loss Priority (CLP), and 8-bit Header Error Control (HEC).
Before transmitting byte streams, the ATM network needs to set up an end-to-end virtual connection, namely, a Virtual Path (VP) with a VPI or a Virtual Connection (VC) with a VCI. The VP is allocated, and occupies a certain number of network resources. Only the virtual connection within a VP needs to occupy resources of a VP dynamically through signaling. Besides, cell loss is inevitable in an ATM network. A CLP can be set for a cell. When the network is congested, the cell whose CLP is set to “1” may be discarded first.
Table 2 shows the format of an 802.1-based Ethernet frame, in which a Destination Address (DA), Source Address (SA), frame Length (LEN), payload type (TYPE), Frame Check Sequence (FCS), Ethernet user bridge network tag (C-TAG), metropolitan area Ethernet service instance tag (S-TAG), and backbone Ethernet service instance tag (I-TAG) are included. For detailed description about the fields, see the IEEE802.1d, IEEE802.1q and IEEE802.1ah protocol files.
TABLE 2802.1d6 bytes6 bytes2 bytesN bytes4 bytesDASALEN/TYPEPayloadFCS802.1q6 bytes6 bytes4 bytes2 bytesN bytes4 bytesDASAC-TAGLEN/TYPEPayloadFCS802.1ah6 bytes6 bytes4 bytes6 bytesN bytes4 bytesDASAS-TAGI-TAGPayloadFCS
As shown in Table 2, a 4-byte C-TAG contains a 2-byte Type Protocol ID (TPID) and a 2-byte Virtual Local Area Network Tag (VLAN TAG); a 6-byte I-TAG contains a 2-byte TPID and a 3-byte backbone Ethernet instance service identifier (I-SID).
The Pseudo Wire Emulation Edge-to-Edge (PWE3) workgroup of the Internet Engineering Task Force (IETF) is engaged in researching different types of PWE3 on a Packet Switched Network (PSN). The emulated services can be transmitted through a frame relay, Ethernet, ATM or Time Division Multiplex (TDM) dedicated line.
The PWE3 can use a tunnel mechanism on the PSN to simulate the attributes required by a service, in which the tunnel is called a Pseudo Wire (PW). The PWE3 serves to package the Protocol Data Units (PDUs) of a specific service. A PDU contains the data and control information required for simulating specific services. By using a PWE3 mechanism, the operator may transfer all transmission services to an integrated network such as an Internet Protocol/Multi-Protocol Label Switching (IP/MPLS) network. A universal PWE3 package generally contains four parts: header of the PSN, pseudo wire identifier, control word and PDU. For the PWE3 reference model, see the IETF PWE3 rfc3985 protocol document.
The International Telecom Union (ITU) document “ITU-T Y.1411” specifies the format of an ATM cell package of an NNI on an MPLS network.
TABLE 3MPLS tunnel label (4N bytes)PW label (4 bytes)Control word (4 bytes)Payload
The 32-bit structure of the control word in Table 3 is: 2-bit zero information, 14-bit length (LEN) and 16-bit sequence number (SN), where LEN and SN are optional and can be set to zero if they are not selected.
In the process of packaging ATM cells, one ATM cell may be mapped to one Ethernet frame, which is called “one-to-one mode”; or multiple ATM cells are mapped to one Ethernet frame, which is called “N-to-one mode”. The length of an ATM cell is 53 bytes fixedly. Therefore, ATM cells may be packaged in two ways:
single-cell package: one cell is packaged in an MPLS packet; and
package of connected cells: multiple cells are packaged in an MPLS packet. A package of connected cells may be generated by repeating the single-cell package format in the payload area of an Ethernet frame.
Table 4-1 shows the NNI single-cell package format of the one-to-one mode. A package of connected cells of the one-to-one mode can be generated by repeating the single-cell package format of the one-to-one mode in the payload area of an Ethernet frame.
TABLE 4-187654321MODEVCIP00PTICLPVCI (optional)VCI (optional)ATM payload (48 bits)
As shown in Table 4-1, the 1-bit MODE indicates the packaging mode; if MODE=0, the packaging mode is cell packaging; if MODE=1, the packaging mode is frame packaging.
As shown in Table 4-1, the 1-bit VCIP indicates whether a VCI is included in a payload package.
If VCIP=0, it indicates that no VCI is included in a payload package. In this case, the package format contains no 16-bit VCI area shown in Table 4-1.
If VCIP=1, it indicates a VCI is included in a payload package. In this case, the package format contains a 16-bit VCI area, which comes in two circumstances: (i) the first ATM cell is packaged in an Ethernet frame; (ii) the VCI of the currently packaged ATM cell in an Ethernet frame is different from the VCI of the previously packaged ATM cell in the Ethernet frame.
Table 4-2 shows the NNI single-cell package format of the N-to-one mode. A package of connected cells of the N-to-one mode can be generated by repeating the single-cell package format of the N-to-one mode in the payload area of an Ethernet frame. One PW may bear more than one ATM Virtual Channel Connection (VCC) or Virtual Path Connection (VPC). Therefore, a payload package needs to contain a VPI and a VCI.
TABLE 4-287654321VPIVPIVCIVCIVCIPTICLPATM payload (48 bits)
However, the research object of the PWE3 workgroup is limited to PSNs based on IP or MPLS. However, an Ethernet is a connectionless packet-based network, which is different from IP- or MPLS-based networks.
In conventional art, a method for interworking between the Ethernet and the ATM network is provided that refers to the contents about service interworking between the Ethernet and the ATM network in the IEEE 802.1 protocol (802.1d, 802.1q and 802.1ah protocol). The network model of the method in the conventional art is shown in FIG. 1.
As shown in FIG. 1, when the Interworking Function (IWF) between an Ethernet network and an ATM network handles an Ethernet frame or ATM cell, the Ethernet frame or the ATM cell cannot be converted directly on layer 2, but has to be converted through an upper-layer protocol.
The interworking between different types of networks comes in two types: network interworking and service interworking. In network interworking, two terminal networks use the same protocol, and different protocols are used between the central network and the terminal network; and the existence of the network in the central location is transparent to the users at both sides. In service interworking, different protocols may be used between two terminal networks, and peer-to-peer communication is performed between them. However, the peer-to-peer communication requires assistance of the upper-layer protocol.
The conventional art implements conversion between an ATM cell and an Ethernet frame through the processing based on the upper-layer protocol from the perspective of service interworking. The upper-layer protocol over the ATM network is IP protocol, the IWF from the ATM to the Ethernet can strip the IP packet from the ATM cell, package the IP packet into the Ethernet frame and send it to the Ethernet; the IWF from the Ethernet to the ATM can strip the IP packet from the Ethernet frame, package the IP packet into the ATM cell and send it to the ATM network.
The interconnection between two networks from the perspective of service interworking in the conventional art ultimately relies on the conversion between different layers of the Open System Interconnection (OSI) model. That is, in order for the IWF to strip the upper-layer protocol packets from the ATM network or the Ethernet or to package such packets, the IWF needs to learn the type of the upper-layer protocol in addition to the ATM cell format and the Ethernet frame format.